This invention relates to an automatic pressure control device for a hydraulic driving circuit used for driving a plurality of actuators of a construction machine or the like by a single hydraulic pressure source and, especially, to an automatic control device which serves to control a pilot pressure of each changeover valve in the driving circuit to reduce its aperture, thereby reducing a demanded flow rate of each actuator.
Generally, as shown in FIG. 1, a hydraulic actuator driving circuit of this kind includes a single hydraulic pressure source, such as a hydraulic pump 6, for driving a plurality of actuators, for example two actuators 7 and 8, and changeover valves 1 and 2 inserted between an output passage 14 of the hydraulic pump 6 and input passages 7b and 8b of the actuators 7 and 8 for controlling flow rates therebetween. The changeover valves 1 and 2 have three changeover positions 1a, 1b, 1c and 2a, 2b, 2c and pairs of main pilot chambers 1d, 1e and 2d, 2e, respectively. The main pilot chambers 1d, 1e, 2d and 2e are connected respectively through passages 10, 11, 12 and 13 to the outlets of proportional control valves 3a, 3b and 4a, 4b which are alternately controlled by levers 3 and 4 for selectively receiving a hydraulic pressure from a pilot pump 5 to move the changeover valves 1 and 2 from their neutral positions 1b and 2b as shown to either left or right changeover position to drive the actuators 7 and 8 forwards or backwards. The outlet of the hydraulic pump 6 is also connected through a bypass flow rate control valve 9 to a tank T. The bypass flow rate control valve 9 has a spring chamber 9a provided with a spring 9b and the spring chamber 9a is connected through a passage 22 to a shuttle valve 21. The shuttle valve 21 has two (generally, plural) inlets for receiving load pressures of the actuators 7 and 8 through passages 19 and 20 and the changeover valves 1 and 2 and an outlet for delivering the highest of them. The bypass flow rate control valve 9 serves a function of responding to this highest load pressure to drain part of the output pressure of the pump to the tank T. Check valves 15 and 16 and inline flow rate control valves 17 and 18 are inserted between the pump 6 and the changeover valves 1 and 2, respectively.
Although there is no problem in such prior art driving circuit when the total flow rate demanded by the actuators is less than a power limit or the maximum flow rate of the pump 6, there is a problem of an actuator of a lower load being preferentially operated due to a deficiency in the amount of feed when the demanded total flow rate exceeds the power limit of the pump. In order to remove this problem, such an automatic control device has been developed in that the apertures of the changeover valves are reduced for reducing the demanded flow rates of the respective actuators when the demanded total flow rate of the actuators exceeds the power limit of the pump. An example of this kind of device is disclosed in Japanese opened patent gazette No. H1-269704. This control device comprises a pair of counter pilot chambers disposed in both sides of each changeover valve in addition to the main pilot chambers, a pair of passages for connecting the main pilot chambers to the counter pilot chambers in the opposite side of the main pilot chambers, respectively, and a pair of pressure reducing valves inserted in these passages for operating in response to a difference between the output pressure and the highest load pressure of the actuators. When the demanded total flow rate of the actuators exceeds the power limit or the maximum flow rate of the pump in this device, the difference between the output pressure of the pump and the highest load pressure becomes low. This reduction is sensed by the pressure reducing valves which raise the pressures of the counter pilot chambers to move the changeover valves toward their neutral positions to reduce their apertures. Accordingly, it is possible to reduce the demanded total flow rate without substantial reduction in the operation speed of the actuator of low demanded flow rate.
However, this control device has such a disadvantage in that it is complicated in structure and expensive since it uses changeover valves of a special structure having counter pilot chambers and special pressure reducing valves of differential operation type.
Accordingly, an object of this invention is to provide an economical control device of simple structure which can effect a similar operation without use of any valve of such a special structure as above.